1. Field of the Invention
The present invention relates to a constant voltage circuit for generating a constant voltage, and more particularly, to a technique for a circuit capable of generating a constant voltage independent of temperature.
2. Description of the Related Art
FIG. 11 is a diagram showing a configuration of a conventional constant voltage circuit capable of generating a constant voltage independent of temperature. A series circuit of a constant current source 54 and a reference voltage circuit 51, the source and drain of an NPN transistor 53, the power source and ground ends of a differential amplification circuit 52, and a series circuit of resistances R51 and R52, are each connected between an output terminal 50 for outputting an constant voltage Vout and a ground. The constant current source 54 applies a constant current to the reference voltage circuit 51, which in turn generates a predetermined reference voltage VR5 independent of temperature. The reference voltage VR5 is input to the inverting input of the differential amplification circuit 52. The series circuit of the resistances R51 and R52 is a resistor-divider circuit for dividing the constant voltage Vout to generate a fractional voltage VM. The fractional voltage VM is input to the non-inverting input of the differential amplification circuit 52. The differential amplification circuit 52 amplifies a voltage difference between the input reference voltage VR5 and the fractional voltage VM and outputs the result to the gate of the NPN transistor 53.
In the circuit of FIG. 11, when the constant voltage Vout increases, the fractional voltage VM may become higher than the reference voltage VR5, so that the output level of the differential amplification circuit 52 increases. Therefore, the NPN transistor 53 increases a current that flows from the output terminal 50 to the ground so as to decrease the constant voltage Vout. When the constant voltage Vout decreases, the fractional voltage VM may become lower than the reference voltage VR5, so that the output level of the differential amplification circuit 52 decreases. Therefore, the NPN transistor 53 decreases a current flowing from the output terminal 50 to the ground so as to increase the constant voltage Vout. Specifically, the constant voltage Vout is controlled so that the reference voltage VR5 is equal to the fractional voltage VM, thereby making it possible to cause the constant voltage Vout to be independent of temperature and proportional to the predetermined reference voltage VR5 that is independent of temperature.
(Patent Document 1) Japanese Unexamined Patent Application Publication No. H08-185232
However, in the circuit of FIG. 11, the temperature-independent reference voltage VR5 has a fixed voltage value for each set of manufacturing conditions, i.e., the voltage value cannot be freely set. Therefore, in order to allow the constant voltage Vout to be freely set, the series circuit of the resistances R51 and R52 for generating the fractional voltage VM that is input to one of the inputs of the differential amplification circuit 52, needs to be provided as a circuit having a current path different from that of the constant current source 54 that supplies a current to the reference voltage circuit 51 that generates the reference voltage VR5. Therefore, when the current consumption of the constant voltage circuit is reduced, the current consumption of the three circuits, i.e., the constant current source 54, the series circuit of the resistances R51 and R52, and the differential amplification circuit 52, needs to be reduced, and therefore, it is difficult to design low current consumption. Particularly, the current consumption of the series circuit of the resistances R51 and R52 is large since it is connected between the output terminal 50 of the constant voltage Vout and the ground. Therefore, an additional circuit configuration is required to design low current consumption, resulting in a non-negligible influence on manufacturing cost. Also, the circuit scale of the constant current source 54 or the reference voltage circuit 51 needs to be increased so as to cause the circuit reference voltage VR5 to be independent of temperature.